Add namespaces for php storm @see: https://docs.typo3.org/m/typo3/book-extbasefluid/master/en-us/8-Fluid/8-developing-a-custom-viewhelper.html#local-namespace-import-via-syntax
<div xmlns="http://www.w3.org/1999/xhtml"
xmlns:flux="http://typo3.org/ns/FluidTYPO3/Flux/ViewHelpers"
xmlns:v="http://typo3.org/ns/FluidTYPO3/Vhs/ViewHelpers"
xmlns:f="http://typo3.org/ns/fluid/ViewHelpers"
xmlns:xo="http://typo3.org/ns/Ps/Xo/ViewHelpers">
<f:layout name="Default"/>
<f:section name="Main" />
</div>